3GPP and some to come 5G licensed networks will shortly begin trials to offer services in unlicensed bands. License Assisted Access for Long Term Evolution (LAA-LTE or LAA), as the first example, has recently been demonstrated at Mobile World Congress in March of 2015 using the 5 GHz band. Field trials will start later this year with product rollouts planned for 2016 and 2017.
Unlicensed band advocates have expressed concerns that Wi-Fi, currently the dominant technology deployed in the 5 GHz band, may be adversely affected by LAA-LTE. Concerns over channel sharing have been raised—where Wi-Fi advocates have stated that LAA-LTE, which does not employ channel sharing or listen before talk etiquette functions as Wi-Fi does, may take over the channel and starve Wi-Fi users of the unlicensed band radio channel resource.
To this end, LAA-LTE advocates have simulated, and executed proof of concept demonstrations showing that Wi-Fi and LAA-LTE do co-exist on the unlicensed band channel, and that Wi-Fi typically operates to consume the unused channel time not used by LAA-LTE transmissions.
LAA-LTE advocates have proposed that operators manage the LAA-LTE channel utilization by setting the maximum transmitter duty cycle, so that in environments where there are few Wi-Fi users, LAA-LTE will have more capacity, and in environments where there are many Wi-Fi users, LAA-LTE users will have less bandwidth.
Wi-Fi advocates have responded that such channel sharing arrangements leave all decision making to operators, who may act without full regard to the needs of Wi-Fi users. The LAA-LTE users have responded with many different possible solutions which have the potential for truly fair sharing including, but not limited to, using Wi-Fi receivers in the LAA-LTE radio to monitor Wi-Fi activity, thereby providing insight as to the level of Wi-Fi activity. Moreover, it is expected that the Wi-Fi receivers would perform the dual function of radar signal detection to meet Dynamic Frequency Selection (DFS) requirements—a regulatory function necessary to operate in parts of the 5 GHz unlicensed band. The same Wi-Fi receivers could operate in “promiscuous mode”—a well-known mode of Wi-Fi transceiver operation which is used by packet sniffers applications such as “Omnipeak” to view Over-The-Air (OTA) Wi-Fi data packet traffic for testing and debug purposes. Promiscuous mode is also used by Wi-Fi security applications such as “Air Defense” to monitor OTA traffic in search of Wi-Fi transceivers attempting hostile actions such as Denial of Service attacks, or hackers attempting to gain access to a private Wireless Local Area Network (WLAN).
In the context of LAA-LTE, Wi-Fi transceivers operating in promiscuous mode may be used to monitor Wi-Fi OTA traffic, and to determine the number of idle and active Wi-Fi devices, so that information may be fed back to the LAA-LTE radio algorithms to adaptively change LAA-LTE sharing controls.
For example, if an LAA-LTE radio used an embedded Wi-Fi transceiver, and detected 10 different Wi-Fi beacons from 5 different physical Wi-Fi Access Points (APs) (note that APs may transmit multiple beacons—typically up to 8, but occasionally up to 16), as well as probes packets from 25 different STAtion (STA) clients such as laptops and smart phones, but only two of the devices were actively transferring data, the Wi-Fi transceiver could provide feedback to the LAA-LTE radio indicating the “average” channel loading (from all of the transmitted beacons, and random probe packets and probe responses), as well as the analysis that there were two Wi-Fi devices transferring significant amounts of data. The LAA-LTE radio could then use this information to set an appropriate sharing threshold, accounting for the background “average” OTA utilization (which may be 10% of the airtime), plus the fact that there were two active Wi-Fi devices, so that the LAA-LTE receiver would allocate for itself 30% of the available airtime, calculated as (100% -10% average loading)/(3 devices). This would ensure that the LAA-LTE radio would fairly share the air with the other two Wi-Fi devices, while not overly excessively penalizing itself by assuming that all 10 visible Wi-Fi beacons were “actively” sharing the channel.
This proposal, as explained above, is easily understood, but fails in several aspects explained below.
Incorporating a Wi-Fi receiver into an LAA-LTE receiver does not provide a TTM (time-to-market) solution. Wireless network customers are excited for LAA-LTE as a means to massively increase LTE connectivity leveraging unlicensed spectrum. LTE networks currently form the largest wireless networks globally, and are well-supported by industry. They are therefore currently the fastest means to address international demands of a global economy untethering itself at an unprecedented pace, while doubling data traffic every two years. While Wi-Fi has addressed this requirement for some indoor spaces, specifically residential and enterprise, the same is not true for outdoor national networks.
Any LAA-LTE solution should be as simple as adding another band—a process which designers of LTE networks have completed many times in the past to address different frequency requirements globally. Therefore, moving to 5 GHz should be as simple as re-designing for a new LTE frequency variant, which typically can be accomplished in a period of 12-18 months. Incorporating a Wi-Fi chipset in current LAA-LTE receivers would likely not be possible within that time period and would likely result in a much longer development cycle.
In addition, an LAA-LTE solution which must include a new protocol stack (to accommodate the additional Wi-Fi chipset) would have a great impact on the entire network affecting all layers of hardware and software up to the network layer. The LTE PHY layer is commonly split by all equipment manufacturers into remote (antenna+RF) and centralized (common equipment/control). The 3GPP interface between these two layers is CPRI—Common Public Radio Interface. The addition of a Wi-Fi receiver would affect remote and common equipment, impact CPRI, and affects all network level Software functions. Achieving industry consensus for such a proposed solution would take many years to achieve, and to rollout.
The proposed solution does not address Federal Communications Commission (FCC) and global regulator mandates addressing how they manage the spectrum. The role of regulators such as the FCC is not to define radio solution implementations, but rather, how devices must perform to ensure that the spectrum resources are well managed.
The 5 GHz unlicensed band called the Unlicensed National Information Interchange (U-NII) is managed by the FCC as being a digitally modulated band, with specified Conducted Power and Effective Isotropic Radiated Power (EIRP) levels, Power Spectral Density (PSD) levels, bandwidths, and out-of-band emissions. Requirements for the detection of “radar” in terms of their power signatures, durations, periodicities, and “chirp” characteristics are defined by the regulators, and required radio actions defined, such as channel scanning times, and times when devices must keep off the channel.
Regulators define monitoring requirements in terms of low level radio parameters, and they do not define how the receiver should work, or protocol related requirements of receivers.
Regulators take the position that their role is to manage the spectrum, while not defining the solution, as this enables industry and competition to arrive at the best solution, without unnecessary requirements.
Although the U-NII band is predominantly used by Wi-Fi devices, regulations do not mandate Wi-Fi devices. Regulators have conducted many studies on Wi-Fi, but they will not force the industry to make Wi-Fi the only solution, as this would restrict industry and technology advancement.
This is not to say that the industry has been pushing regulators to make the U-NII bands Wi-Fi bands, but that in itself is not likely to happen as wireless untethering, with the Internet of Things (IoT), an infrastructure to wirelessly connect things such as toasters, watches, and car keys in very early days but looming on the horizon, coupled with the anticipated massive rollout of the 3GPP and 5G evolutionary products from cellular equipment manufacturers and service providers. In other words, the unlicensed bands are likely to remain unlicensed—free for all to use, without regulated protocols, but with limited PHY level rules to ensure that devices can operate without adversely affecting or being affected by other wireless devices operating in the same band.
The incorporation of a Wi-Fi receiver solution is therefore arguably for the short-term and does not address the next generation of IoT & 3GPP devices. It focuses on co-existence with the Wi-Fi protocol stack, a current problem but only one of potentially many other co-existence problems to arise in the future in the U-NII or other shared bands.
The industry proposed solution has been stated to “work for Wi-Fi” but in reality does not work for MIMO (multiple-input multiple output) and MU-MIMO (multi-user MIMO), commonly used in 802.11n and 802.11ac Wi-Fi chipsets. MIMO is the default operation of laptops and phablets operate with 2×2, 3×3 and soon 4×4 MIMO and MU-MIMO. All of the new 5 GHz smart phones are switching to 2×2 MIMO to double 5 GHz throughput, while improving spectrum efficiency. The solution assumes that these MIMO devices can be demodulated by the Wi-Fi receiver on the LAA-LTE radio, but in actual fact, in many cases they cannot. First, in order to demodulate an N×N signal, the Wi-Fi receiver in the LAA-LTE radio must have N antennas. It is highly unlikely that LAA-LTE systems will use greater than 2 antennas for most applications, while indoor enterprise Wi-Fi systems have all switched to 4×4 MIMO support. Additionally, Wi-Fi radios employ MIMO and MU-MIMO to increase OTA efficiency and throughput, and they achieve these higher rates by optimizing SNR on a client by client basis, using RAA (Rate Adaptation Algorithms) designed to operate at the highest possible bit rate for maximum efficiency. It is difficult, if not impossible for an LAA-LTE radio with N-antennas to demodulate all (N×N) OTA packets, since it is highly unlikely that the LAA-LTE radio will receive sufficient SINR for the packet to be demodulated. Therefore, these packets, even though they may have good RSSI, will appear as noise, making their statistics invisible in a solution depending on counting them.
The industry proposed solution may also have longer term ethical and legality issues, as OTA packets are demodulated and information collected about “BSSIDs” and MAC addresses.
Finally, the industry proposed solution may not work for non-standard Wi-Fi channels, or for Wi-Fi signals which are on adjacent channels. For example, LAA-LTE is expected to use channel 32 (5160 MHz) which is not a standard Wi-Fi channel, yet will see interference from channel 34 (5170 MHz) which is a Wi-Fi channel. The proposed solution would actually require multiple receivers spaced at 5 MHz offsets to be able to look for Wi-Fi interference across the various 5 MHz channel offsets.
Accordingly, to address some or all of the drawbacks noted above, it would be desirable to avoid the use of a Wi-Fi receiver in LAA-LTE radios for co-existence over an unlicensed band.